Environmental Problems, Their Causes, and Sustainability 1

technologies or greatly improve their energy efficiencyover the next few decades.What Is Net Energy Efficiency? HonestEnergy AccountingNet energy efficiency is a measure of how muchuseful energy we get from an energy resourceafter subtracting the energy used and wastedin making the energy available.Recall that the only energy that really counts is netenergy (p. 354). The net energy efficiency of a systemused to heat your house, for example, is determined bythe efficiency of each step in the energy conversion forthe entire system.Figure 18-6 shows the net energy efficiency forheating two well-insulated homes. One is heated withelectricity produced at a nuclear power plant, transportedby wire to the home, and converted to heat (electricresistance heating). The other is heated passively:direct solar energy enters through high-efficiency windowsfacing the sun and strikes heat-absorbing materialsthat store the heat for slow release.This analysis shows that converting the high-qualityenergy in nuclear fuel to high-quality heat at severalthousand degrees in the power plant, converting thisheat to high-quality electricity, transmitting the electricityto users, and using the electricity to provide lowqualityheat for warming a house to only about 20°C(68°F) is very wasteful of high-quality energy. Althoughthe last step of using the incoming electricity toproduce heat is 100% efficient, the numerous stepsneeded to get the electricity to the house waste enormousamounts of energy. Burning coal or any fossil fuelat a power plant to supply electricity and transmitting itlong distances to heat water or space is also inefficient.This example illustrates two general principles forsaving energy. First, keep the number of steps in an energyconversion process as low as possible. Each time we convertenergy from one form to another or transmit it,some useful energy is almost always lost. Second,strive to have the highest possible energy efficiency for eachstep in an energy conversion process.18-2 WAYS TO IMPROVE ENERGYEFFICIENCYHow Can We Save Energy in Industry?Cogenerate, Buy New Motors, and UseEfficient LightingIndustries can save energy and money by producingboth heat and electricity from an energy sourceand by using energy-efficient electric motors andlighting.Some industries save energy and money by using cogeneration,or combined heat and power (CHP) systems.In such a system two useful forms of energy (such assteam and electricity) are produced from the same fuelUraniummining(95%)Uranium processingand transportation(57%)Powerplant(31%)Transmissionof electricity(85%)Resistanceheating(100%)Uranium100%95%54%17% 14% 14%WasteheatWasteheatWasteheatWasteheatElectricity from Nuclear Power PlantSunlight100%Passive SolarWindowtransmission(90%)Wasteheat90%Figure 18-6 Comparison of net energy efficiency for two types ofspace heating. The cumulative net efficiency is obtained by multiplyingthe percentage shown inside the circle before each step bythe energy efficiency for that step (shown in parentheses). So100 0.95 95%; 95 0.57 54%; and so on. Because of thesecond law of thermodynamics, in most cases the greater thenumber of steps in an energy conversion process, the lower its netenergy efficiency. About 86% of the energy used to provide spaceheating by electricity produced at a nuclear power plant iswasted. If the additional energy needed to deal with nuclearwastes and to retire highly radioactive nuclear plants after theiruseful life is included, then the net energy yield for a nuclear plantis only about 8% (or 92% waste). By contrast, with passive solarheating, only about 10% of incoming solar energy is wasted.382 CHAPTER 18 Energy Efficiency and Renewable Energy